Reversing CRC32
本文关键字：CRC32 Reversing | 更新日期: 2023-09-27 18:24:29
我找到了一个反向CRC32的代码，但我不知道它是如何工作的，因为我在编程方面不是很好，我刚刚开始。我只想比较两个文件，旧的和新的，然后在新的修复中，CRC32在文件末尾添加了4个字节，所以这两个文件将具有相同的CRC32。这是代码，在C#中：
public class Crc32
{
public const uint poly = 0xedb88320;
public const uint startxor = 0xffffffff;
static uint[] table = null;
static uint[] revtable = null;
public void FixChecksum(byte[] bytes, int length, int fixpos, uint wantcrc)
{
if (fixpos + 4 > length) return;
uint crc = startxor;
for (int i = 0; i < fixpos; i++) {
crc = (crc >> 8) ^ table[(crc ^ bytes[i]) & 0xff];
}
Array.Copy(BitConverter.GetBytes(crc), 0, bytes, fixpos, 4);
crc = wantcrc ^ startxor;
for (int i = length - 1; i >= fixpos; i--) {
crc = (crc << 8) ^ revtable[crc >> (3 * 8)] ^ bytes[i];
}
Array.Copy(BitConverter.GetBytes(crc), 0, bytes, fixpos, 4);
}
public Crc32()
{
if (Crc32.table == null) {
uint[] table = new uint[256];
uint[] revtable = new uint[256];
uint fwd, rev;
for (int i = 0; i < table.Length; i++) {
fwd = (uint)i;
rev = (uint)(i) << (3 * 8);
for (int j = 8; j > 0; j--) {
if ((fwd & 1) == 1) {
fwd = (uint)((fwd >> 1) ^ poly);
} else {
fwd >>= 1;
}
if ((rev & 0x80000000) != 0) {
rev = ((rev ^ poly) << 1) | 1;
} else {
rev <<= 1;
}
}
table[i] = fwd;
revtable[i] = rev;
}
Crc32.table = table;
Crc32.revtable = revtable;
}
}
}
以下是一些PHP代码，它们也可以满足您的需求我花了很多时间试图为合法目的做这件事所以我希望它能帮助
像这样的功能可以用于：
- 总是有一个单独的散列来验证文件（例如，特定类型的所有文件都返回一个公共crc、版本控制等，没有数据库）
- 修改哈希的分布方式（例如，分布式计算密钥平衡）
- crc32的反向原始4字节
要在线测试crc32字符串，可以使用：http://www.lammertbies.nl/comm/info/crc-calculation.html
请参阅http://www.reversing.be/article.php?story=20061209172953555这是从哪里改编的由于我不是原作者，原作者在他的文章上花了很多时间在很大程度上帮助
以下是可用于修改给定文件的PHP代码，如下所示：
<?php
//Calculate the original file's CRC32
$DesiredCRC32 = hexdec(hash_file('crc32b','originalcleanfile.ext'));
//Calculate the newfile's current CRC
$CurrentIncorrectCRC32 = hexdec(hash_file('crc32b','newupdatedfile.ext'));
//Generate a 4 byte string that, if appended to the newfile will result in the original CRC32
echo modCRC($DesiredCRC32,$CurrentIncorrectCRC32);
// I should note that the original guy that posted this has some incorrect calcs on his page
// but if you read carefully and understand the math, you can pick up where it's incorrect
// and learn a thing or two (I beleive last time I tested this code it was accurate)
// if you wanted to do something via the CLI you could use something like:
parse_str(implode('&', array_slice($argv, 1)), $_GET);
// and grab the files w/
$file1 = $_GET['a'];
//... etc.
//then you could run it via something like this:
//c:'php'php -f somefile.php a=c:'original.txt b=c:'new.txt c=c:'newwithcrc.txt
public static function modCRC($ExistingCRC, $DesiredCRC){
$crc32lookup = array(
0x00000000, 0x77073096, 0xEE0E612C, 0x990951BA, 0x076DC419, 0x706AF48F, 0xE963A535, 0x9E6495A3,
0x0EDB8832, 0x79DCB8A4, 0xE0D5E91E, 0x97D2D988, 0x09B64C2B, 0x7EB17CBD, 0xE7B82D07, 0x90BF1D91,
0x1DB71064, 0x6AB020F2, 0xF3B97148, 0x84BE41DE, 0x1ADAD47D, 0x6DDDE4EB, 0xF4D4B551, 0x83D385C7,
0x136C9856, 0x646BA8C0, 0xFD62F97A, 0x8A65C9EC, 0x14015C4F, 0x63066CD9, 0xFA0F3D63, 0x8D080DF5,
0x3B6E20C8, 0x4C69105E, 0xD56041E4, 0xA2677172, 0x3C03E4D1, 0x4B04D447, 0xD20D85FD, 0xA50AB56B,
0x35B5A8FA, 0x42B2986C, 0xDBBBC9D6, 0xACBCF940, 0x32D86CE3, 0x45DF5C75, 0xDCD60DCF, 0xABD13D59,
0x26D930AC, 0x51DE003A, 0xC8D75180, 0xBFD06116, 0x21B4F4B5, 0x56B3C423, 0xCFBA9599, 0xB8BDA50F,
0x2802B89E, 0x5F058808, 0xC60CD9B2, 0xB10BE924, 0x2F6F7C87, 0x58684C11, 0xC1611DAB, 0xB6662D3D,
0x76DC4190, 0x01DB7106, 0x98D220BC, 0xEFD5102A, 0x71B18589, 0x06B6B51F, 0x9FBFE4A5, 0xE8B8D433,
0x7807C9A2, 0x0F00F934, 0x9609A88E, 0xE10E9818, 0x7F6A0DBB, 0x086D3D2D, 0x91646C97, 0xE6635C01,
0x6B6B51F4, 0x1C6C6162, 0x856530D8, 0xF262004E, 0x6C0695ED, 0x1B01A57B, 0x8208F4C1, 0xF50FC457,
0x65B0D9C6, 0x12B7E950, 0x8BBEB8EA, 0xFCB9887C, 0x62DD1DDF, 0x15DA2D49, 0x8CD37CF3, 0xFBD44C65,
0x4DB26158, 0x3AB551CE, 0xA3BC0074, 0xD4BB30E2, 0x4ADFA541, 0x3DD895D7, 0xA4D1C46D, 0xD3D6F4FB,
0x4369E96A, 0x346ED9FC, 0xAD678846, 0xDA60B8D0, 0x44042D73, 0x33031DE5, 0xAA0A4C5F, 0xDD0D7CC9,
0x5005713C, 0x270241AA, 0xBE0B1010, 0xC90C2086, 0x5768B525, 0x206F85B3, 0xB966D409, 0xCE61E49F,
0x5EDEF90E, 0x29D9C998, 0xB0D09822, 0xC7D7A8B4, 0x59B33D17, 0x2EB40D81, 0xB7BD5C3B, 0xC0BA6CAD,
0xEDB88320, 0x9ABFB3B6, 0x03B6E20C, 0x74B1D29A, 0xEAD54739, 0x9DD277AF, 0x04DB2615, 0x73DC1683,
0xE3630B12, 0x94643B84, 0x0D6D6A3E, 0x7A6A5AA8, 0xE40ECF0B, 0x9309FF9D, 0x0A00AE27, 0x7D079EB1,
0xF00F9344, 0x8708A3D2, 0x1E01F268, 0x6906C2FE, 0xF762575D, 0x806567CB, 0x196C3671, 0x6E6B06E7,
0xFED41B76, 0x89D32BE0, 0x10DA7A5A, 0x67DD4ACC, 0xF9B9DF6F, 0x8EBEEFF9, 0x17B7BE43, 0x60B08ED5,
0xD6D6A3E8, 0xA1D1937E, 0x38D8C2C4, 0x4FDFF252, 0xD1BB67F1, 0xA6BC5767, 0x3FB506DD, 0x48B2364B,
0xD80D2BDA, 0xAF0A1B4C, 0x36034AF6, 0x41047A60, 0xDF60EFC3, 0xA867DF55, 0x316E8EEF, 0x4669BE79,
0xCB61B38C, 0xBC66831A, 0x256FD2A0, 0x5268E236, 0xCC0C7795, 0xBB0B4703, 0x220216B9, 0x5505262F,
0xC5BA3BBE, 0xB2BD0B28, 0x2BB45A92, 0x5CB36A04, 0xC2D7FFA7, 0xB5D0CF31, 0x2CD99E8B, 0x5BDEAE1D,
0x9B64C2B0, 0xEC63F226, 0x756AA39C, 0x026D930A, 0x9C0906A9, 0xEB0E363F, 0x72076785, 0x05005713,
0x95BF4A82, 0xE2B87A14, 0x7BB12BAE, 0x0CB61B38, 0x92D28E9B, 0xE5D5BE0D, 0x7CDCEFB7, 0x0BDBDF21,
0x86D3D2D4, 0xF1D4E242, 0x68DDB3F8, 0x1FDA836E, 0x81BE16CD, 0xF6B9265B, 0x6FB077E1, 0x18B74777,
0x88085AE6, 0xFF0F6A70, 0x66063BCA, 0x11010B5C, 0x8F659EFF, 0xF862AE69, 0x616BFFD3, 0x166CCF45,
0xA00AE278, 0xD70DD2EE, 0x4E048354, 0x3903B3C2, 0xA7672661, 0xD06016F7, 0x4969474D, 0x3E6E77DB,
0xAED16A4A, 0xD9D65ADC, 0x40DF0B66, 0x37D83BF0, 0xA9BCAE53, 0xDEBB9EC5, 0x47B2CF7F, 0x30B5FFE9,
0xBDBDF21C, 0xCABAC28A, 0x53B39330, 0x24B4A3A6, 0xBAD03605, 0xCDD70693, 0x54DE5729, 0x23D967BF,
0xB3667A2E, 0xC4614AB8, 0x5D681B02, 0x2A6F2B94, 0xB40BBE37, 0xC30C8EA1, 0x5A05DF1B, 0x2D02EF8D
);
//More Description:
//Want to append bytes: X Y Z W
//Take for register a3 a2 a1 a0 (a is the 'already calculated' crc for string)
//Note that a3 is the most significant byte and a0 the least.
// f is the wanted new CRC byte string, and xyzw is the modification pad
/*
I'll show it a little different way:
a0 + X =(1) points to b3 b2 b1 b0 in table
a1 + b0 + Y =(2) points to c3 c2 c1 c0 in table
a2 + b1 + c0 + Z =(3) points to d3 d2 d1 d0 in table
a3 + b2 + c1 + d0 + W =(4) points to e4 e3 e2 e1 in table
b3 + c2 + d1 + e0 =f0
c3 + d2 + e1 =f1
d3 + e2 =f2
e3 =f3
(1) (2) (3) (4)
(figure 4)
This is reversed in the same way as the 16bit version. I shall give an example with real values. For the table values use the CRC-32 table in the appendix.
Take for CRC register before, a3 a2 a1 a0 -> AB CD EF 66
Take for CRC register after, f3 f2 f1 f0 -> 56 33 14 78 (wanted value)
Here we go:
First byte of entries entry value
e3=f3 =56 -> 35h=(4) 56B3C423 for e3 e2 e1 e0
d3=f2+e2 =33+B3 =E6 -> 4Fh=(3) E6635C01 for d3 d2 d1 d0
c3=f1+e1+d2 =14+C4+63 =B3 -> F8h=(2) B3667A2E for c3 c2 c1 c0
b3=f0+e0+d1+c2=78+23+5C+66=61 -> DEh=(1) 616BFFD3 for b3 b2 b1 b0
Now we have all needed values, then
X=(1)+ a0= DE+66=B8
Y=(2)+ b0+a1= F8+D3+EF=C4
Z=(3)+ c0+b1+a2= 4F+2E+FF+CD=53
W=(4)+d0+c1+b2+a3=35+01+7A+6B+AB=8E
(final computation)
*/
$ExistingCRC = ~$ExistingCRC;
$DesiredCRC = ~$DesiredCRC;
//a3a2a1a0
$a3 = (($ExistingCRC & 0xFF000000) >> 24) & 0x000000FF;
$a2 = ($ExistingCRC & 0x00FF0000) >> 16;
$a1 = ($ExistingCRC & 0x0000FF00) >> 8;
$a0 = ($ExistingCRC & 0x000000FF);
//f3f2f1f0
$f3 = (($DesiredCRC & 0xFF000000) >> 24) & 0x000000FF;
$f2 = ($DesiredCRC & 0x00FF0000) >> 16;
$f1 = ($DesiredCRC & 0x0000FF00) >> 8;
$f0 = ($DesiredCRC & 0x000000FF);
//echo "Existing CRC:" . dechex($ExistingCRC) . " DesiredCRC:" . dechex($DesiredCRC) . "<BR>";
//echo "a3:" . dechex($a3) . " a2:" . dechex($a2) . " a1:" . dechex($a1) . " a0:" . dechex($a0) . "<br>";
//echo "f3:" . dechex($f3) . " f2:" . dechex($f2) . " f1:" . dechex($f1) . " f0:" . dechex($f0) . "<br>";
//capture e3/e2/e1/e0/(4) values
for ($i = 0; $i < 256; $i++)
{
if (($crc32lookup[$i] & 0xFF000000 ) == ($DesiredCRC & 0xFF000000)){
$e3 = (($crc32lookup[$i] & 0xFF000000) >> 24) & 0x000000FF;
$e2 = ($crc32lookup[$i] & 0x00FF0000) >> 16;
$e1 = ($crc32lookup[$i] & 0x0000FF00) >> 8;
$e0 = ($crc32lookup[$i] & 0x000000FF);
$four = $i;
break;
}
}
//echo "e3:" . dechex($e3) . " e2:" . dechex($e2) . " e1:" . dechex($e1) . " e0:" . dechex($e0) . "<br>";
//$d3=f2+e2 =33+B3 =E6 -> 4Fh=(3) E6635C01 for d3 d2 d1 d0
$d3 = $f2^$e2; //lookup $d3 and assign the values for d2/d1/d0/(3)
for ($i = 0; $i < 256; $i++)
{
if (($crc32lookup[$i] & 0xFF000000 ) == ($d3 << 24)){
$d2 = ($crc32lookup[$i] & 0x00FF0000) >> 16;
$d1 = ($crc32lookup[$i] & 0x0000FF00) >> 8;
$d0 = ($crc32lookup[$i] & 0x000000FF);
$three = $i;
break;
}
}
//echo "d3:" . dechex($d3) . " d2:" . dechex($d2) . " d1:" . dechex($d1) . " d0:" . dechex($d0) . "<br>";
//c3=f1+e1+d2 =14+C4+63 =B3 -> F8h=(2) B3667A2E for c3 c2 c1 c0
$c3 = $f1^$e1^$d2;
for ($i = 0; $i < 256; $i++)
{
if (($crc32lookup[$i] & 0xFF000000 ) == ($c3 << 24)){
$c2 = ($crc32lookup[$i] & 0x00FF0000) >> 16;
$c1 = ($crc32lookup[$i] & 0x0000FF00) >> 8;
$c0 = ($crc32lookup[$i] & 0x000000FF);
$two = $i;
break;
}
}
//echo "c3:" . dechex($c3) . " c2:" . dechex($c2) . " c1:" . dechex($c1) . " c0:" . dechex($c0) . "<br>";
//b3=f0+e0+d1+c2=78+23+5C+66=61 -> DEh=(1) 616BFFD3 for b3 b2 b1 b0
$b3 = $f0^$e0^$d1^$c2;
for ($i = 0; $i < 256; $i++)
{
if (($crc32lookup[$i] & 0xFF000000 ) == ($b3 << 24)){
$b2 = ($crc32lookup[$i] & 0x00FF0000) >> 16;
$b1 = ($crc32lookup[$i] & 0x0000FF00) >> 8;
$b0 = ($crc32lookup[$i] & 0x000000FF);
$one = $i;
break;
}
}
/*
Now we have all needed values, then
X=(1)+ a0= DE+66=B8
Y=(2)+ b0+a1= F8+D3+EF=C4
Z=(3)+ c0+b1+a2= 4F+2E+FF+CD=53
W=(4)+d0+c1+b2+a3=35+01+7A+6B+AB=8E
(final computation)
*/
$X = $one ^ $a0;
$Y = $two ^ $b0^$a1;
$Z = $three ^ $c0^$b1^$a2;
$W = $four ^ $d0 ^ $c1 ^ $b2^$a3;
//echo "Four:" . dechex($four) . " Three:" .dechex($three)." Two:".dechex($two)."One:" .dechex($one) . "<br>";
//echo "X:" . dechex($X) . " Y:" . dechex($Y) . " Z:" . dechex($Z) . " W:" . dechex($W) . "<br>";
return chr($X) . chr($Y) . chr($Z) . chr($W);
}
?>
您想知道它是如何工作的，或者如何使用它吗？
如果是晚一点，则从代码签名：
public void FixChecksum(byte[] bytes, int length, int fixpos, uint wantcrc)
似乎您将第二个文件的内容放入了一个数组中（在修复的末尾添加了4个字节），并将其作为bytes参数传递。将该数组的长度作为length参数，将偏移量作为fixpos参数（在本例中为length-4），将所需的CRC作为wantcrc参数，通过计算第一个文件的CRC，可以获得该值。
FixChecksum方法似乎在您提供的偏移量处写入您提供的数组中的4字节修复。调用FixChecksum之后，只需要将结果写入第二个文件。